The embodiments described herein relate generally to steam turbines, and more particularly, to methods and systems for reducing a swirl effect of a cooling flow for a rotor of the steam turbine.
As steam turbines rely on higher steam temperatures to increase efficiency, steam turbines are fabricated to withstand the higher steam temperatures so as not to compromise the useful life of the turbine. During a typical turbine operation, steam flows from a steam source through a housing inlet and substantially parallel to an axis of rotation along an annular hot steam path. Typically, turbine stages are positioned within the steam path such that the steam flows through vanes and blades of subsequent turbine stages. The turbine blades may be secured to a plurality of turbine wheels, where each turbine wheel is coupled to, or is formed, integral with the rotor shaft for rotation therewith. Alternatively, the turbine blades may be secured to a drum type turbine rotor rather than individual wheels, wherein the drum is formed integrally with the shaft.
At least some turbine blades include an airfoil that extends radially outward from a substantially planar platform, and a root portion that extends radially inwardly from the platform. The root portion may include a dovetail or other means to secure the blade to the turbine wheel of the turbine rotor. In general, during operation, steam flows over and around the turbine blade, which are subject to high thermal stresses. These high thermal stresses may limit the service life of the turbine blades, the wheel, and/or the rotor. More particularly, as steam temperatures increase, the rotor materials may experience creep and rupture. Conventional steam turbines may use materials that are more temperature resistant to increase the operating life and performance of the rotor. However, these materials may increase the cost of fabrication of the turbine rotor. Some steam turbines may inject cooling steam from an intermediate pressure stage towards the rotor. Typical cooling steam, however, may have a swirl effect that may affect the heat transfer from the rotor and/or negatively affect rotor operation.